A detailed understanding of existing mortars is essential in informing and assisting the specification of replacement or repair materials in historic buildings. It can also contribute to understanding a building's construction history. Mortar analysis is often required for listed buildings consent. We provide:
- Identification of the constituents and proportions of all mortars, plasters and renders
- Assessment of hydraulicity
- Determination of non-hydraulic lime; hydraulic lime; natural cements; Roman cements; early cements and Portland cement; cement/lime blends; gypsum and gypsum/lime blends
- Identification of sand, aggregates and additives.
The procedure comprises:
- Visual observations and inspection of the sample.
- Moisture content of the sample as-received.
- Phenolphthalein carbonation test.
- Chemical 'dissolution' tests in accordance with BS4551:2005 +A2:2013 to determine: proportions of acid-soluble calcium, magnesium, iron, sulphur, aluminium, acid and alkali soluble silica, cloride, and acid insolubles; Reaction comments; Presence of acid-soluble calcareous aggregates.
- Identification and microscopic examination by stereo microscope (Leica S9i) of insoluble sands, aggregates and additives.
- Dry-sieving to determine fines proportion.
- Interpretation of results in a detailed report which includes: binder and aggregate type; stereo microscope images of the insoluble material; hydraulicity of binder and evidence of pozzolanic reactions; calculation of original volumetric mix proportions; suggestions for a matching repair or replacement mortar mix recipe on a 'like-for-like' basis and possible explanations for failure, poor performance or deleterious reactions (if applicable)
Figure 1. Insolubles (>63µm) from a Roman bedding mortar. View under stereomicroscope at magnification x10.
Thin Section Analysis
Thin section analysis can be used to augment the standard analysis and gives further information on the sample. This analysis comprises:
- The Standard Analysis Procedure
- Optical microscopy by thin section analysis with compound microscope (Leica DM2700M). In transmitted, plane and cross polarised light, to determine: pore structure, porosity, micro-cracking; textural/spatial interrelationships between components; the presence of lime nodules; mineral identification of sand and aggregates. In reflected light, to identify components of hydraulic binder and determine potential mortar layers.
Additional Investigative Techniques
Further additional instrumental techniques can be employed if necessary.
- SEM (Scanning Electron Microscopy) combined with EDX (Energy Dispersive X-ray spectroscopy)
- XRD (X-ray diffraction) & XRF (X-ray fluorescence) micro analysis
- DTA (Differential Thermal Analysis)
- Water soluble salt analysis
Figure 2. Medieval lime mortar. Microscopy of a thin section under transmitted plane polarised light (magnification x25). The blue colour represents resin impregnated pores.
Figure 3. Medieval lime mortar (Same image as Fig. 2).
Microscopy of a thin section under transmitted cross-polarised light (magnification x25) showing quartz grains.
- A representative sample of at least 40g is required where possible, and contextual information is always useful. Smaller amounts of material can be assessed where it is not practical or appropriate to obtain more but may limit what testing is possible.
- Samples may be delivered to our offices: Horchester Farm, Holywell, Dorchester DT2 0LL.
- Mortar analysis normally takes three to four weeks.
- Please note that two-coat plaster, for example, is two samples if both coats are to be tested.
Historic mortars overtime
The great majority of historic mortars were based on lime, but traditional limes varied depending on many factors including the particular limestone burnt, kiln design, fuel type and kiln temperature. Binder:aggregate ratios vary and pozzolanic reactions may have occurred, or deleterious processes taken place. Aggregate type and grading, fines proportion and void ratio also affect performance. The picture becomes complex from 1780 with the development of Roman cement, artificial hydraulic limes, early cements and eventually Portland cement by about 1840. Learn more by reading our articles.
Figure 4. Insolubles (sand >63µm) from a lime plaster. View under stereomicroscope at x30.